1. Field of the Invention
The invention described in the present specification relates to a panel structure of an electroluminescent (EL) display panel driven and controlled by an active matrix driving system. Incidentally, the invention proposed in the present specification has an aspect as an electronic device including the EL display panel.
2. Description of the Related Art
FIG. 1 shows a circuit block configuration typical of an organic EL panel of an active matrix driving type. As shown in FIG. 1, the organic EL panel 1 includes a pixel array section 3, a writing control line driving section 5 as a driving circuit for driving the pixel array section 3, and a horizontal selector 7. Incidentally, the pixel array section 3 has a pixel circuit 9 disposed at each of intersections of signal lines DTL and writing control lines WSL.
An organic EL element is a current light emitting element. The organic EL panel therefore adopts a driving system that controls gradation by controlling an amount of current flowing through an organic EL element corresponding to each pixel. FIG. 2 shows one of simplest circuit configurations of pixel circuits 9 of this kind. This pixel circuit 9 includes a sampling transistor T1, a driving transistor T2, and a storage capacitor Cs.
The sampling transistor T1 is a thin film transistor for controlling the writing of a signal voltage Vsig corresponding to the gradation of the corresponding pixel to the storage capacitor Cs. The driving transistor T2 is a thin film transistor for supplying a driving current Ids to an organic EL element OLED on the basis of a gate-to-source voltage Vgs determined according to the signal voltage Vsig retained by the storage capacitor Cs. In the case of FIG. 2, the sampling transistor T1 is formed by an n-channel type thin film transistor, and the driving transistor T2 is formed by a p-channel type thin film transistor.
In the case of FIG. 2, the source electrode of the driving transistor T2 is connected to a power supply line to which a fixed potential (power supply potential Vcc) is applied, and the driving transistor T2 operates in a saturation region at all times. That is, the driving transistor T2 operates as a constant-current source supplying the organic EL element OLED with the driving current having a magnitude corresponding to the signal voltage Vsig. At this time, the driving current Ids is given by the following equation.Ids=k·μ·(Vgs−Vth)2/2
where μ is the mobility of majority carriers of the driving transistor T2, Vth is the threshold voltage of the driving transistor T2, and k is a coefficient given by (W/L)·Cox, where W is channel width, L is channel Length, and Cox is a gate capacitance per unit area.
Incidentally, it is known that in the case of the pixel circuit of this configuration, the drain voltage of the driving transistor T2 is changed with temporal change in I-V characteristic of the organic EL element as shown in FIG. 3. However, because the gate-to-source voltage Vgs is held constant, an amount of current supplied to the organic EL element is unchanged, so that light emission luminance can be held constant.
The following are documents relating to organic EL panel displays adopting an active matrix driving system.
Japanese Patent Laid-open No. 2003-255856
Japanese Patent Laid-open No. 2003-271095
Japanese Patent Laid-open No. 2004-133240
Japanese Patent Laid-open No. 2004-029791
Japanese Patent Laid-open No. 2004-093682